8.2 Using ifnames to List Conditionals

ifnames can help you write configure.ac for a software
package. It prints the identifiers that the package already uses in C
preprocessor conditionals. If a package has already been set up to have
some portability, ifnames can thus help you figure out what its
configure needs to check for. It may help fill in some gaps in a
configure.ac generated by autoscan (see autoscan Invocation).

ifnames scans all of the C source files named on the command line
(or the standard input, if none are given) and writes to the standard
output a sorted list of all the identifiers that appear in those files
in #if, #elif, #ifdef, or #ifndef
directives. It prints each identifier on a line, followed by a
space-separated list of the files in which that identifier occurs.